How to texture Dragon/Reptile for later import into UE4?
Well I won't give you an easy why to do it but I can tell you some basics that you can do. 
You are wanting to make a texture that resembles the skin of a reptile. First look up reptiles and look at there skin.
Then you can either try to hand paint this in gimp or you can use an image that has authorized the use of the image to be used by anyone. You can grab a piece of the image and then you can look up how to make a seamless texture which will let you repeat it as much as you want to create the effect of reptilian skin. 

If you are having trouble finding tutorials on this you can look up either how to paint a texture and apply it to reptilian skin or you can look up how to manipulate an image and use it to make into a texture. 

This might also help you: https://www.youtube.com/watch?v=TLj2bDa2qno